1. Field of the Invention
The present invention relates to a rear projection television set having a projection cathode ray tube, mirrors which reflects the light from the tube onto a screen, and a screen that receives the light reflected by the mirror and passes it therethrough, and which the image projected on the screen is viewed from the other side of the screen.
2. Prior Art
FIG. 7 shows a prior art rear projection television set of this type. A projector lens 3 is located in front of the frame of the projection cathode ray tube 2 housed in a cabinet 1. The light projected from the tube 2 is condensed by the projector lens 3, reflected by mirrors 4 and 5, and then received by a screen 6. The screen 6 is generally of the construction in which a Fresnel lens is superposed to a lenticular sheet formed of a plurality of cylindrical lenses which extend vertically and are disposed with narrow spaces therebetween. The light is reflected by the mirror 5 onto the Fresnel lens and passes through the lenticular sheet opposite the viewers.
The light emitted from the screen includes two kinds of lights; one is directly viewed as an image by the viewer and the other is an undesired light Lu that has adverse effects on the picture quality. The unwanted light Lu is emitted downwards at an angle of .THETA..sub.1 with respect to a line normal to the screen as shown in FIG. 9.
As shown in FIG. 8, a major portion Ld of the light incident upon the Fresnel lens 7 passes through the screen into the viewer's eyes as a desired image. A minor portion of the incident light is reflected by the lens surface (left side in FIG. 8) of Fresnel lens 7. The rest (not shown) is reflected by the back surface (right side in FIG. 8) of Fresnel lens. Further, the light reflected by the lens-surface is again reflected by the back surface of Fresnel lens. The term undesired light is used to refer to the light Lu reflected by the lens surface of the Fresnel lens. Part of the undesired light Lu is reflected by the mirror 5 and then reaches the Fresnel lens. The light Lu is emitted from the Fresnel lens with a downward refraction angle of .THETA..sub.1. This angle is small when the distance A between the Fresnel lens and the mirror 5 is large as shown in FIG. 9. It should be noted that the undesired light Lu is emitted over an area H on the screen which is approximately below a center line C. Thus, when the screen is approximately as high as viewer's eyes, the undesired light will not go into the viewer's eyes and therefore the contrast of the screen 6 is unaffected. Conventional rear projection television sets have a large depth D as shown in FIG. 7, that is, the distance A between the Fresnel lens 7 and mirror 5 is relatively long. Thus, the undesired light emitted from the screen will not reach the viewer's eyes even if the angle .THETA..sub.1 is small. In recent years, the trend is that a short depth of the cabinet is required of the rear projection television sets. Shorter the depth of the cabinet is, the shorter the distance A between the Fresnel lens 7 and mirror 5 becomes. As shown in FIG. 10, as the distance A becomes shorter, the undesired light contains more amount of light reflected by the mirror 5 onto the Fresnel lens 7. The conventional Fresnel lenses have long condensing distances in the range of 8-10 meters and therefore the light through the Fresnel lens 7 has a small exit angle .THETA..sub.1, thus deteriorating the contrast of the screen 6. This undesired light is emitted from almost the entire surface H of the screen. Increasing screen gain (brightness) prevents the deterioration of screen contrast but causes narrower viewing angles.